1. Field of the Application
The present invention relates to plasma arc torches. More specifically, the invention is concerned with electric arc devices for heating and treating materials, wherein the arc discharge is acted upon by an external magnetic field. This invention also has to do with plasma arc torches with a magnetically-controlled arc, which are used for melting and treating materials with a plasma arc.
2. Description of the Prior Art
There is known a metal melting plasma arc torch which comprises a cathode, a cathode holder, a cooled body and a nozzle.
The most important working member of the torch head in such torches is the nozzle, since the latter is exposed to high temperatures in the course of operation. The interior passage of the nozzle is cooled with a flow of water which is directed along the cooled wall thereof and is passed through an annular recess. By reason of high heat loads acting on the nozzle interior passage, a flow of water must be supplied at a high rate and under great pressure.
There are also known plasma arc torches of indirect action, which are provided with a centrally disposed electrode surrounded by a nozzle. In a flow of gas passing through the nozzle interior there burns an electric discharge which is acted upon by external magnetic field with the purpose of creating pinch effect and raising the plasma jet temperature (cf. U.S. Pat. No. 2,945,119), and also with the purpose of deflecting the plasma jet away from the nozzle axis (cf. Japanese Pat. No. 6958), or to dissipate the heat flux of the plasma jet (cf. Japanese Pat. No. 3261), as well as to enable other operations.
A magnetic field is most frequently used for the purpose of causing the arc to rotate about the internal surface of the plasma arc torch nozzle, this permitting the arc burning voltage and the torch efficiency to be increased, and the nozzle erosion to be substantially inhibited.
British Patent Specification No. 966,103 describes, for example, a plasma arc torch (plasma generator) which comprises a body having a cooled nozzle attached thereto and accommodating a centrally disposed electrode. A plurality of electromagnetic coils fitted over the nozzle coaxially therewith are fixed and protected from heat by means of a fixing ring member in threaded engagement with the nozzle. The arc rotation in the nozzle interior passage at various sections thereof is achieved by alternately energizing the magnetic coils. The disadvantage of the torch construction according to the patent referred to above lies in that the magnetic coils are poorly protected from the heat radiated from the fixing member, or from the particles of the material being treated. For example, by using this type of the torch for making granules, the magnetic coils are rapidly put out of order due to failure in the insulation caused by the heat radiation or by incandescent particles of said material.
An object of the invention is to provide such a plasma arc torch head which will permit the power of a torch to be enhanced by means of improving the conditions of cooling the surfaces subject to maximum heating in the process of operation.
Another object of the invention is to provide a plasma arc torch head which will be sufficiently reliable and durable in operation.
Still another object of the invention is to provide a plasma arc torch head which will have sufficient durability when operated in the conditions of plasma jets of complex configuration, controlled by an electromagnetic field.
It is also an object of the invention to provide an improved plasma arc torch head featuring the above-mentioned advantages attained without introducing costly constructional modifications, by relatively simple and reliable means.
These and other objects of the invention are accomplished by the provision of a plasma arc torch head comprising a hollow tubular body through the interior of which passes a flow of plasma-forming gas and wherein are axially arranged an electrode holder and an electrode having its free end surrounded by a cooled nozzle formed with double walls interconnected at the end thereof by means of a solid bridging member and forming the end face of the torch head. The plasma arc torch head according to the invention is characterized by that the head body is formed with coolant inlet ducts which are oriented so as to enable the flow of the incoming coolant to be directed normally to the cooled surfaces of the portions of the nozzle surface subject to maximum heating in the process of generation of plasma jet.
Such constructional arrangement of the plasma arc torch head of the invention permits cooling of the nozzle areas exposed to severe heating to be substantially improved due to directing the flow of incoming coolant directly against the surface being cooled and thus ensuring considerably higher rate of its cooling in contrast to the flow of coolant smoothly passing over the entire surface of said nozzle.
According to one embodiment of the invention, the coolant inlet ducts are formed along the periphery of the torch head body in parallel to its central axis and extend short of the end face of the torch head, the coolant exhaust ducts alternating with the coolant inlet ducts which pass into radial channels extending radially to cooled cylinder-shaped wall of the nozzle.
Such structural arrangement makes it possible to improve the removal of heat from the nozzle surfaces subject to high heat loads with the nozzle wall being exposed to the burning of the plasma-forming arc.
According to another embodiment of the invention, electromagnetic coils are mounted concentrically externally of the nozzle to thereby allow control of the plasma jet at the outlet of the torch, characterized by that said electromagnetic coils are inclosed in a protective housing flush-mounted with the nozzle in the area of the end face thereof, with an annular plug being fitted between the protective housing and the tubular body of the torch head, the coolant inlet ducts extending over the entire length of the electromagnetic coils and normally to said bridging member between the two walls of the nozzle.
Such structural arrangement enables the electromagnetic coils to be protected from destruction and turn-to-turn short-circuiting caused by high temperatures of the plasma jet, given out by the material being treated owing to sufficient cooling of the nozzle end face and by providing a ring-shaped plug.
Such construction of the plasma generator permits the electromagnetic coils to be isolated from the effect of high temperatures of the ambient atmosphere while making use of the cooled portion of the nozzle and of the fixing plug member intended to close the space with the magnet coils enclosed therein and used to fixedly attach the nozzle to the body. Being disposed within an easy reach, the coils are readily changeable. In addition, it is easy to lead out the terminals, since the enclosure containing the coils is not sealed.
According to still another embodiment of the invention, the fixing plug member is made of ferromagnetic material in the form of a ring threaded at both faces differing from each other in at least one feature such as pitch or direction of thread.
Such structural arrangement makes it possible to enhance the effect of the electromagnetic coils on the plasma jet by stepping up intensity of the magnetic field thereof.